Spread-spectrum MEMS self-test system and method

ABSTRACT

A MEMS sensor includes a micro-electromechanical structure, a detection circuit, and a self-test circuit to test the health of the MEMS sensor during runtime operations. The self-test circuit is configured to inject into the micro-electromechanical structure a plurality of injected test signals that are broad-band frequency-varying frequency signals, which are based on spread spectrum based modulation. The injected test signals may a magnitude that is below an observable threshold of the sensor signal as well as a test-signal bandwidth that overlaps with a substantial portion of the sensor bandwidth, including the stimulus of interest.

FIELD OF THE INVENTION

The present invention relates generally to a micro-electromechanicalsystem (MEMS) device, more particularly a MEMS sensor having spreadspectrum modulation self-testing capabilities.

BACKGROUND OF THE INVENTION

Inertial-based MEMS sensors, such as MEMS gyroscopes and accelerometers,are generally being used in greater quantities and in greater numbers ofcontrols applications in automotive and vehicle systems. Theseapplications may include, for example, usage in electronic stabilitycontrols (ESC), anti-lock braking systems (ABS), and supplementalrestraint systems (SRS). Such sensors are also being used in variousother applications, for example, in medical systems, such as inmechanized prostheses. In these and other MEMS applications,self-testing capability (also referred to as health monitoring) may be abeneficial if not a necessary feature to ensure that the MEMS sensor isoperating properly within the application.

To ensure the proper operations of the MEMS sensor, those in the arthave developed MEMS sensors with in-the-field self-testing capability.This self-test function generally entails injecting a known stimulusinto the micro-electromechanical structure of a MEMS sensor during theinitialization or startup operation of the sensor. As such, suchself-test function may not observe a fault during the runtime operation.

Those in the art also have employed test function during the runtimeoperation of the sensor. The test stimulus of such runtime operationtypically consists of an impulse response or a waveform that is appliedoutside the bandwidth or frequency of the stimulus of interest, which isgenerally a physical attribute intended to be observed by the MEMSsensors. As such, the test may provide an incomplete test of the MEMSsensor, as a portion of the potential bandwidth for measurement may beallotted for the testing function. Additionally, since the test isapplied to a limited frequency range, the self-test function may, forexample, fail to detect a defect that manifests in a specific frequencyrange. In particular, MEMS gyroscopes employed in ESC systems, forexample, have been found to have certain linear and torsional vibrationfrequency that may not be measured concurrently with the operation ofthe self-test function.

Moreover, the bandwidth of a MEMS sensor is generally constrained by thedesign limitations and material properties of themicro-electromechanical structures. Thus, increasing the bandwidth ofthe MEMS sensor may require a redesign of the sensor, which may not becost effective or possible.

SUMMARY OF EXEMPLARY EMBODIMENTS

In one embodiment there is provided a MEMS sensor having a runtimeself-test circuit, wherein the MEMS sensor comprises amicro-electromechanical device that outputs, during runtime operation, adevice output signal; a detection circuit configured to detect, duringthe runtime operation of the micro-electromechanical device, a stimulusproduced by the micro-electromechanical system in a sensor bandwidth ofthe device output signal, the stimulus having a magnitude above apredetermined detection threshold of the detection circuit, thedetection circuit further configured to produce a sensor output signalbased on the detected stimulus; and a self-test circuit configured to(1) inject, during the runtime operation of the micro-electromechanicaldevice, a self-test signal into a signal path of themicro-electromechanical device including a MEMS structure involved withgeneration of the stimulus in the sensor bandwidth of the device outputsignal, wherein the self-test signal is a spread-spectrum signal havinga test-signal bandwidth that overlaps at least a portion of the sensorbandwidth; (2) detect a test signal component in the test-signalbandwidth of the device output signal based on a reference test signalcorresponding to the injected self-test signal, the test signalcomponent have a magnitude below the detection threshold of thedetection circuit; and (3) produce a test output signal indicating astatus for the micro-electromechanical system based on the test signalcomponent and the reference test signal.

In another embodiment there is provided a self-test circuit for amicro-electromechanical system that outputs, during runtime operation, adevice output signal including a stimulus in a sensor bandwidth of thedevice output signal, the stimulus having a magnitude above apredetermined detection threshold. The self-test circuit comprises aself-test signal generator configured to inject, during runtimeoperation of the micro-electromechanical device, a self-test signal intoa signal path of the micro-electromechanical device including a MEMSstructure involved with generation of the stimulus in a sensor bandwidthof the device output signal, wherein the self-test signal is aspread-spectrum signal having a test-signal bandwidth that overlaps atleast a portion of the sensor bandwidth; a self-test signal detectorconfigured to detect a test signal component in the test-signalbandwidth of the device output signal based on a reference test signalcorresponding to the injected self-test signal, the test signalcomponent have a magnitude below the detection threshold of thedetection circuit; and a controller configured to produce a test outputsignal based on the detected test signal component.

In various alternative embodiments, the self-test circuit may beconfigured to continuously inject the self-test signal into themicro-electromechanical device during the runtime operation of themicro-electromechanical device. The self-test circuit may include apseudo-random number sequence source configured to provide apseudo-random number sequence and a modulation circuit configured tomodulate the pseudo-random number sequence and a self-test magnitudereference to produce a modulated signal and to inject the modulatedsignal into the micro-electromechanical device. The self-test magnitudereference may be a constant direct-current (DC) source. Thepseudo-random number sequence source may comprise a pseudo-random numbergenerator. The pseudo-random number sequence source may comprises amemory in which is stored the pseudo-random number sequence. Thebandwidth of the self-test signal may be equal to the sensor bandwidth,less than the sensor bandwidth, or greater than the sensor bandwidth.The bandwidth of the self-test signal may extend below the sensorbandwidth and/or may extend above the sensor bandwidth. The self-testcircuit may also include a demodulation circuit configured to detect thetest signal component by demodulating the device output signal with thereference test signal in-phase with test signal component. The self-testcircuit may include a low-pass filter to extract the test signalcomponent from the device output signal. The self-test circuit mayinclude a delay circuit to produce the reference test signal in-phasewith the test signal component. The self-test circuit may include amemory to provide the reference test signal. The demodulation circuitmay comprise a multiplier to combine the test signal component with thein-phase reference test signal.

In certain embodiments, the self-test circuit may be configured todetect the test signal component in the test-signal bandwidth of thedevice output by (i) correlating the sensor signal with thepseudo-random number sequence in phase with the sensor signal to producea correlation signal and (ii) comparing the correlation signal to atleast one of a pre-defined correlation threshold or a pre-defined signalenergy threshold. The pre-defined energy threshold may be establishedbased on the relationship fc*STM2, wherein fc is a frequency value ofthe injected self-test test signal and STM is an average power of aperiod spread of the injected self-test signal.

In any of the above embodiments, the self-test circuit may be configuredto inject the self-test signal according to at least one of afrequency-hopping spread spectrum modulation, a direct-sequence spreadspectrum (DSSS) modulation, a time-hopping spread spectrum modulation,or a chirp spread spectrum modulation. The micro-electromechanicaldevice may include an inertial sensor, a sound sensor, a pressuresensor, and/or other MEMS sensor or device.

In another embodiment there is provided a method of evaluating thestatus of a MEMS sensor during runtime, the MEMS sensor having amicro-electromechanical device that outputs, during a runtime operation,a device output signal, the MEMS sensor further having a detectioncircuit configured to detect, during the runtime operation of themicro-electromechanical device, a stimulus produced by themicro-electromechanical system in a sensor bandwidth of the deviceoutput signal, the stimulus having a magnitude above a predetermineddetection threshold of the detection circuit, the detection circuitfurther configured to produce a sensor output signal based on thedetected stimulus. The method involves injecting, during runtimeoperation of the micro-electromechanical device, a self-test signal intoa signal path of the micro-electromechanical device including a MEMSstructure involved with generation of a stimulus in a sensor bandwidthof a device output signal, wherein the self-test signal is aspread-spectrum signal having a test-signal bandwidth that overlaps atleast a portion of the sensor bandwidth; detecting a test signalcomponent in the test-signal bandwidth of the device output signal basedon a reference test signal corresponding to the injected self-testsignal, the test signal component have a magnitude below the detectionthreshold of the detection circuit; and producing a test output signalbased on the detected test signal component.

Additional embodiments may be disclosed and claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of embodiments will be more readily understood byreference to the following detailed description, taken with reference tothe accompanying drawings, in which:

FIG. 1 schematically shows an in-field self-test MEMS sensor accordingto an illustrative embodiment;

FIG. 2 schematically shows a continuous self-test circuit of a MEMSsensor according to an alternate embodiment;

FIG. 3 schematically shows a MEMS system according to an embodiment;

FIG. 4A schematically shows a MEMS system of FIG. 3 according to anembodiment;

FIG. 4B illustrates frequency characteristics of the various self-teststimulus signals according to an embodiment;

FIG. 4C shows frequency components of a sensor stimulus and self-teststimulus according to a conventional self-test system;

FIG. 5 schematically shows a MEMS system according to an embodiment;

FIG. 6 is a plot illustrating the effect on spectrum power of varyinglength of the number sequence used to spread the self-test signal;

FIG. 7 is a plot illustrating the effect of the magnitude of theself-test signal on the power spectrum of the pseudo-random numbergenerator;

FIG. 8 is a plot illustrating the effect of the chipping frequency onthe pseudo-random number generated sequence;

FIG. 9 is a plot illustrating the power of the spectrum of thepseudo-random number generated sequence of FIG. 8;

FIGS. 10A-B illustrate plots of the synchronization of the varyingfrequency self-test signal at the receiver;

FIG. 11 is a plot of an output of a low-pass filter with different tapconfiguration;

FIG. 12 schematically illustrates an example of a MEMS sensor withself-test capabilities as known in the art; and

FIG. 13 is a flow chart of a method according to an illustrativeembodiment.

It should be noted that the foregoing figures and the elements depictedtherein are not necessarily drawn to consistent scale or to any scale.Unless the context otherwise suggests, like elements are indicated bylike numerals.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

As used herein, the term “self-test” refers to injecting a known teststimulus into a sensor, e.g., via an internal testing component of thesensor, and then measuring and analyzing a corresponding output todetermine whether the sensor is operating within a specified set ofoperations (i.e., working/not working or passed/failed). Theterm“self-test” is interchangeably used herein with “health monitoring.”

As used herein, the term “runtime” refers to an intended mode ofoperation. In the context of sensors, for example, the runtime operationincludes a period of sensor operation when the sensor is sensing thestimulus of interest.

As user herein, the term “continuous” refers to an on-going operationand/or without interruption.

There is a benefit in having a MEMS sensor with continuous self-testingcapabilities that does not have to trade-off its effective sensingbandwidth with the self-test function. There is also a benefit in havinga MEMS sensor that operates across a greater region of or across theentire bandwidth range of the sensor.

In exemplary embodiments, a MEMS sensor includes a self-test circuitthat performs self-testing operations within a portion of the sensingbandwidth of the sensor, including up to the entire range thereof. Thetesting further provides a test stimulus having a magnitude below anobservable threshold of the sensor. As such, the testing does notinterfere with the runtime measurement and may be performed concurrently(e.g., continuously) during the runtime operation of the sensor whilenot requiring a dedicated portion of the bandwidth of the MEMS sensor tooperate. Thus, the exemplary embodiment may provide a comprehensivetesting scheme in evaluating up to the entire bandwidth of the sensor.In addition to a more comprehensive test, embodiments generally providean increased sensing bandwidth for a given mechanical design.

To provide such a testing function, the exemplary embodiments employ aspread spectrum test stimulus. Although those in the art have employedspread spectrum modulation in other testing methodologies, such as inreflectometry for the testing of faulty cables, such methodologies arenon-analogous art due to a different underlying mode of operation. Forexample, rather than merely characterizing impedance of a solid staticstructure as with reflectometry, testing of a MEMS sensor generallyentails characterizing the dynamic operations of intricate components inthe sensor.

Herein, the inventor has recognized that the self-testing of MEMSsensors can be accomplished using spread spectrum based modulation.Specifically, the inventors have discovered that a test signal may beinjected at a signal level below an observable threshold of the MEMSsensor because the coding modulation may provide sufficient signal forthe test signal to be detected. In conjunction therewith, the inventorinferred that the coupling between the dynamic operation of themicro-electromechanical structure and the injected test signal mayprovide information about the micro-electromechanical structure,including, for example, that the structure is or is not operating asintended. Specifically, an inference may be made that the structure ofthe MEMS sensor when operating as intended allows for the modulated testsignal to be retrieved, whereas a faulty structure would distort orimpede the modulated test signal.

Put differently, exemplary embodiments employ a coding modulation thatprimarily operates in an information domain that is weakly linked to thephysical domain. The exemplary embodiments may advantageously use thisrelationship by using a weak stimulus in the physical domain, but onewhich is strong in the information domain. As such, the stimulus may beconfigured to not be observable by the sensor in the physical domain.However, the strong information component that is weakly coupled to thephysical domain may nevertheless be amplified to allow information to betransmitted therethrough.

To do so, exemplary embodiments may, for example, inject multitudes ofweak electrical signal into the physical structure of the MEMS sensor.These weak electrical signals individually produce mechanical responsesthat are not generally observable by the sensor to affect observation ofthe stimulus of interest. However, when the multitudes of signals areevaluated together over a period time, a correlation develops among theinjected signals. This correlation forms the basis of the inference.Specifically, if the physical structure is operating as intended, acertain degree of correlation is expected. And, when the physicalstructure is not operating properly, a lesser correlation may form asthe defect in the physical structure (e.g., mechanical and/or electricalstructure) would affect the responses produced by the injected testsignal. As a result, exemplary embodiments may employ any of varioustypes of spread-spectrum-based modulation.

FIG. 1 is a schematic block diagram showing a MEMS sensor 100 having aspread-spectrum-based runtime self-test circuit such as for continuousself-test, according to one illustrative embodiment. The MEMS sensor 100includes a MEMS device 102 that is configured to respond, or issensitive, to an intended stimulus 104 (also referred herein as astimulus of interest) corresponding to a physical attribute such as, forexample, a change in inertia, acceleration, mechanical deformation,sound waves, pressure, centrifugal force, gravitational force, electricforce, magnetic force, thermal gradient, or electromagnetic force. TheMEMS device 102 may be configured to sense the physical attribute, e.g.,exhibited as a capacitance, inductance, or resistance value or a changethereof, and generate a device output signal 108 corresponding thereto.For example, the MEMS device 102 may have a movable mass capacitivelycoupled to a sense electrode that generate a varying device outputsignal 108 based on the position of the movable mass. The device outputsignal 108 may be generated in other ways using other types of MEMSstructures (e.g., capacitively coupled, mechanically coupled,piezoelectric, etc.) in a wide variety of MEMS device types (e.g.,gyroscopes, accelerometers, microphones, switches, etc.).

The MEMS sensor 100 further includes a detection circuit 106 thatoperatively couples with the MEMS device 102 to sense, via the deviceoutput signal 108, a physical attribute corresponding to a stimulus 104exerted on the MEMS device 102. During the runtime operation of the MEMSsensor 100, the detection circuit 106 processes the device output signal108 to produce a sensor output signal 110 corresponding to the stimulus104 (e.g., a sensor output signal indicative of rotation rate, linearacceleration, an acoustic signal, etc.), where the device output signal108 has a bandwidth that includes the observable bandwidth of the MEMSdevice 102. As such, the device output signal 108 may have a bandwidthsufficient to observe the stimulus of interest.

The MEMS sensor 100 further includes a self-test circuit 112. Theself-test circuit 112 is configured to inject a self-test signal 114into at least one circuit path of the MEMS device 102 including a MEMSmechanical or electromechanical structure. The self-test circuit 112 mayinclude a signal generator 116 that generates the self-test signal 114.The signal generator 116 may include a pseudo-random number sequencegenerator that generates a binary sequence (e.g., a sequence of ‘0’ and‘1’) having broad frequency components. For example, a pseudo-randomnumber sequence generator may include a linear feedback shift register(LFSR). The signal generator 116 may generate a frequency-varying signalhaving any of various types of distributions, such as, for example, aGaussian distribution (i.e., normal), an asymmetric distribution (i.e.,skewed), or a uniform distribution. The bandwidth of the self-testsignal 114 may correspond to the distribution of the frequency of thepseudo-random number sequence; thus the distribution may be configuredto cover a specific frequency range. The pseudo-random number sequencemay be generated during runtime operations or may be stored to beretrieved during runtime operation. Herein, the term“pseudo-random”generally refers to being related to or having been generated by adefinite nonrandom computational process. The pseudo-random numbersequence may be used as, or may form the basis of, the self-test signal114.

As such, the self-test signal 114 may be a broad-band frequency-varyingsignal having a bandwidth that overlaps with at least a portion of thesensor bandwidth up to and including the entire bandwidth and may have abandwidth that extends above and/or below the sensor bandwidth (e.g.,the self-test signal bandwidth may be wider than the sensor bandwidth).Herein, the term “broad-band” refers to the wide bandwidthcharacteristic of the signal. As such, the self-test signal 114 may varyaccording to any of various types of spread spectrum techniques, suchas, for example, frequency-hopping spread spectrum (FHSS),direct-sequence spread spectrum (DSSS), time-hopping spread spectrum(THSS), or chirp spread spectrum (CSS). The self-test signal 114 mayfurther have a magnitude below a signal level observable by thedetection circuit 106, including, for example, the noise-floor thereof.

The self-test circuit 112 may inject the self-test signal 114 accordingto any of various methods. For example, in certain MEMS sensors, theMEMS sensor includes a driver circuit that places a fixed or varyingelectrical signal on a MEMS structure or MEMS circuit path (e.g., aresonator, accelerometer proof mass, microphone diaphragm, driveelectrodes, sense electrodes, etc.). As such, the self-test circuit 112may, for example, inject the self-test signal 114 into the drivercircuit of the MEMS sensor. Alternatively, the self-test circuit 112 mayinclude a secondary driver circuit to operate independent of ordependent with the driver circuit, where the secondary driver circuitapplies the self-test signal 114 to the MEMS device 102.

The self-test circuit 112 may include a self-test signal detector 118configured to detect spread-spectrum test signal components in thedevice output signal 108 based on a reference test signal 115. Thereference test signal 115 may be the self-test signal 114 or anothersignal, such as, for example, a delayed version of the self-test signal114, an expected output characteristic of the signal such as a power orcorrelation value, or other reference signal.

To do the detection, the self-test signal detector 118 may receive thereference test signal 115 and may compare the reference test signal 115with, or operate on, the test signal components of device sensor signal108. For example, the self-test signal detector 118 may include ademodulation circuit (not shown) having a multiplier to demodulate thetest signal components with the reference test signal 115. The referenceself-test signal 115 may be delayed to synchronize the two signals. Assuch, the self-test signal detector 118 may include a delay filterhaving a delay constant corresponding to the signal propagation timethrough the MEMS device 102. Alternatively to a demodulation circuit,the self-test signal detector 118 may employ various signal or powercorrelation filters to determine the correlation between the test signalcomponents of the device sensor signal 108 and the self-test signal 114.The self-test circuit 112 may produce a test output 130 providing astatus indication for the MEMS sensor 100, e.g., a pass/fail status or adegree of confidence or operation based on the amount of correlationbetween the test signal components and the reference test signal.

The self-test circuit 112 may continuously inject the self-test signal114. Alternatively, the injection may be periodic or based on a triggercondition.

As stated above, the pseudo-random number sequence may be generatedduring runtime operations or may be stored to be retrieved duringruntime operation. FIG. 2 is a schematic block diagram showing a MEMSsensor 200 having a spread-spectrum-based runtime self-test circuit suchas for continuous self-test, according to another illustrativeembodiment. Here, rather than generating a pseudo-random numbersequence, the self-test circuit 212 includes a memory module 202 havinga pseudo-random number sequence stored therein. The self-test circuit212 may retrieve the stored pseudo-random number sequence from thememory module 202 and use the stored number sequence for generating theself-test signal 114.

To detect the detected test signal, the self-test circuit 212 may employa self-test signal detector 118 substantially as described above. Forexample, the self-test signal detector 118 may retrieve the referencetest signal 115 from the memory module 202. Alternatively, the self-testcircuit 212 may include a second memory module 204 having stored thereina reference test signal 115. As such, the self-test signal detector 118may obtain the reference test signal 115 from the second memory module204.

FIG. 3 schematically shows an implementation of a MEMS sensor 100according to an illustrative embodiment. The sensor includes a MEMSsensor die 302 and a signal processing application-specific integratedcircuit (ASIC) 304. The MEMS sensor die 302 may be configured to observea stimulus 104, such as, for example, a rate of rotation oracceleration, a sound source, etc. The MEMS sensor die 302 may includevarious mechanical and electromechanical structures (e.g., resonator,proof mass, microphone diaphragm, drive electrodes, sense electrodes,etc.).

The signal processing ASIC 304 may include the detection circuit 106 andthe self-test circuit 112. The signal processing ASIC 304 produces thesensor output 110, such as, for example, a measured rate of rotation oracceleration, an audio output signal, etc. The signal processing ASIC304 also produces a test output 130. The test output may indicate a passor a fail status of the MEMS sensor die 302. The signal processing ASIC304 may provide the self-test signal 114 to the MEMS sensor die 302 andreceive the device output signal 108. The MEMS sensor die 302 and signalprocessing ASIC 304 may be implemented on a single die.

FIG. 4A schematically shows a MEMS-ASIC system of FIG. 3 according to anillustrative embodiment. FIG. 4A is described in conjunction with FIG.4B, which illustrates example frequency characteristics of self-testsignals in the MEMS-ASIC system according to the illustrativeembodiment. In this example, the MEMS-ASIC system 400 includes a MEMSinertial sensor, configured with a MEMS gyroscope sensor 402. In thesensor 402, a reference vibration is applied to a micro-mechanicalstructure therein, and a measurable stimulus corresponding to a rotationstimulus 404 is produced through the Coriolis effect when the sensor isrotated. The rotation stimulus 404 may be expressed as x(t) where tcorresponds to time. The MEMS gyroscope sensor 402 may include othercircuits, such as a detection circuit to detect the rotation stimulus404.

The MEMS-ASIC system 400 may include a self-test signal injectionportion that may include a pseudo-number generator 406 and a multiplier408. The pseudo-number generator 406 may generate a pseudo-numbersequence 410. The multiplier 408 may modulate the pseudo-number sequence410 with a self-test magnitude reference 412 to produce the self-testsignal 114 to be injected into the sensor 402. The self-test magnitudereference 412 may be a constant direct-current (DC) source.

The self-test signal 114 may be expressed as s(t), as shown inEquation 1. The self-test signal 114, s(t), may be a multiplicationbetween the self-test magnitude reference 412, expressed as STM(t), andthe pseudo-number sequence 410, expressed as PN(t).s(t)=STM(t)*PN(t)  (Equation 1)

The self-test signal 114, s(t), may have a bandwidth that is spread overthe bandwidth BW of the MEMS gyroscope sensor 402. The pseudo-randomnumber sequence generator 406 may generate the pseudo-number sequence410 as a binary sequence between −1 and +1 (or 0 and 1) with a length Land a bandwidth of BW. The bit rate of the pseudo-number sequence 410may establish the bit rate of the self-test signal 114. The bandwidth ofthe self-test signal 114 may be referred to as the chipping frequency.

The self-test magnitude reference 412, STM(t), may be regarded as amessage signal in the context of a communication system and may beexpressed as a scalar value STM when it is constant DC source. The STMor STM(t) is the reference test signal and thus forms the basis of adetected test signal. The MEMS gyroscope sensor 402 may provide outputsignal 414, d(t), which includes information component derived from therotation stimulus 404 and the self-test signal 114, as shown in Equation2.d(t)=x(t)+STM*PN _(sensor)(t−τ)  (Equation 2)

As indicated, x(t) is the rotation stimulus 404 (i.e., the stimulus ofinterest). The self-test signal 114, when propagated through the MEMSsensor, may be expressed as STM*PN_(sensor)(t−τ), where τ is the delayconstant. The signal STM*PN_(sensor)(t−τ) may be regarded as aninterference signal in a spread spectrum analysis.

The MEMS-ASIC system 400 may include a self-test signal detectionportion to detect a detected test signal (i.e., STM(t) or STM) in theoutput signal 414. The self-test signal detection portion may include amultiplier 416 and a delay circuit 418. To do the detection, theself-test signal detection portion may use the multiplier 416 tomultiply the output signal 414, d(t) and the pseudo-number sequence 411in-phase therewith. The delay circuit 418 may delay the generatedpseudo-random number sequence 410 with delay constant τ to generate thein-phase pseudo-number sequence 411, which is referred to asPN_(ref)(t−τ). The delay circuit 418 may have the form e^(s′τ). Theoutput of the multiplier 420 may be expressed as y(t) as shown inEquation 3.y(t)=d(t)*PN _(ref)(t−τ)  (Equation 3)

Equation 3 may be expanded with Equation 2 to provide Equation 4.y(t)=[x(t)+STM*PN _(sensor)(t−τ)]*PN _(ref)(t−τ)  (Equation 4)

A property in pseudo-random number sequence operation is thatmultiplication of identical signal results in an identity function(i.e., a result of ‘1’). As such, if PN_(sensor)(t−τ) and PN_(ref)(t−τ)are identical (i.e., PN_(sensor)(t−τ)=PN_(ref)(t−τ)), the result ofPN_(sensor)(t−τ)*PN_(ref)(t−τ) is “1”. As such, y(t) may be expressed asy(t)=x(t)+STM.

However, where the PN_(sensor)(t−τ) and PN_(ref)(t−τ) are not identical(i.e., PN_(sensor)(t−τ)≠PN_(ref)(t−τ)), the result is a convolution ofPN_(sensor)(t−τ) and PN_(ref)(t−τ). In such a scenario,y(t)=x(t)+STM*PN_(sensor)(t−τ)*PN_(ref)(t−τ).

In the context of testing, the MEMS-ASIC system 400 may infer that whenPN_(sensor)(t−τ) and PN_(ref)(t−τ) are identical, then the MEMSgyroscope sensor 402 is operating as intended as it does not distort,impede, or alter the self-test signal 114 in propagating therethrough.Similarly, the MEMS-ASIC system 400 may infer that when PN_(sensor)(t−τ)and PN_(ref)(t−τ) are not identical, then the MEMS gyroscope sensor 402is not operating as intended. Table 1 summarizes this test condition.

TABLE 1 Condition of Inference of Test MEMS sensor Condition Output ofTest (y(t)) Operating as PN_(sensor)(t − τ) = PN_(ref)(t − τ) y(t) =x(t) + STM intended Not operating as PN_(sensor)(t − τ) ≠ PN_(ref)(t −τ) y(t) = x(t) + STM * intended PN_(sensor)(t − τ) * PN_(ref)(t − τ).

In plot 414A of FIG. 4B, the signal characteristics of the output signal414 are shown. The x-axis is expressed in frequency and the y-axis isexpressed in signal magnitude. As such, plot 414A shows a frequencydistribution 422 of the rotation stimulus 404 and a frequencydistribution 424 of the self-test magnitude reference 412. Therotation-stimulus frequency distribution 424 is shown as havingbandwidth 426 while the STM frequency distribution 424 has a largerbandwidth 428. The bandwidth 426 represents the bandwidth of theMEMS-ASIC system 400 in observing or providing measurement for therotation stimulus 404. The bandwidth 422 represents the bandwidth of thepseudo-random number sequence generated by the pseudo-random numbersequence generator 406. As such, the test-signal bandwidth overlaps witha substantial portion of the sensor bandwidth, which includes thebandwidth of the stimulus of interest. Additionally, the detected testsignal component has a magnitude below the observable threshold of therotation-stimulus.

In plot 420A of FIG. 4B, the signal characteristics of the input andoutput of the multiplier 416 when the MEMS gyroscope sensor 402 isoperating as intended. The plot 420A shows a frequency distribution 430of the self-test signal 114 before and after transmission through theMEMS gyroscope sensor 402. Here, the self-test signal 114 has abandwidth equivalent to the pseudo-random number sequence (e.g.,PN_(sensor)(t−τ) and PN_(ref)(t−τ)). As such, since the MEMS gyroscopesensor 402 is operating as intended, the multiplication results in adirac delta function 432 having a magnitude corresponding to theself-test magnitude reference 412, STM(t).

To extract the detected self-signal STM from the multiplier outputsignal 420, the MEMS-ASIC system 400 may include a low-pass filter 434(referring back to FIG. 4A). The output of the low-pass filter 434 maybe used as a basis for the test output 130 generated by a controller128. As indicated, the test output 130 may provide a pass or fail statusthat indicates the proper or improper operation of the MEMS gyroscopesensor 402. The low-pass filter 434 may be any of various types offilters, including a cascaded integrator comb (CIC) filter.

In plot 436A of FIG. 4B, the signal characteristics of the output 436 ofthe low-pass filter 434 is shown. The plot 310A shows a filteredfrequency distribution 438 of the rotation-stimulus frequencydistribution 424 (as shown in plot 414A). Due to the filter, therotation stimulus information has been substantially removed, and thedetected test signal (i.e., the dirac delta function 432) remains. Here,the detected test signal may be regarded as the dirac delta function432.

The MEMS-ASIC system 400 may include a controller 128 to provide thetest output 130. The controller 128 may compare the detected test signalto the reference test signal. Here, the reference test signal may adirac-delta function. As such, a presence of the dirac-delta functionindicates a pass status and the lack of the function indicates a failstatus.

Alternatively, the controller may compare the detected test signal to areference signal that is a threshold (i.e., C_(Threshold)) or anequivalent thereto. As such, the pass condition may bePN_(sensor)(t−τ)*PN(t−τ)>C_(Threshold), and the inverse is applied forthe fail condition.

According to an alternative embodiment, rather than using a dirac-deltafunction as the reference test signal, the reference test signal may bean expected signal characteristic based of the self-test signal,including, for example, the average power.

For example, where the self-test magnitude reference 412 is a DC sourceSTM, the average power (Power_(average)) may be expressed asPower_(average)=f_(c)*STM², where f_(c) is the chipping frequency. Aself-test signal may be expected to have all the power within abandwidth BW, where a band-pass filter is employed in the signal pathwith bandwidth BW (e.g., f0/16). As such, when the length N of thepseudo-random number sequence is N=2^(r)−1 with chipping frequencyf_(c), the sequence repeats itself, in time domain, with a time periodof T=N*(1/f_(c)). Thus, the frequency may be expressed as 1/T=f_(c)/N.As a result, the average power of the self-test signal with magnitudeSTM may be expressed as STM=Energy in T/T. Equation 4 shows the averagepower of this periodic varying frequency self-test signalPower_(average)=Energy∥T∥/T  (Equation 4)

Energy∥T∥ may be expressed as a sum of squared number sequence. Thus,the Power_(average) may be expressed as follows in equation 5, where thenumber sequence x[n] is generated over the time period, T.Power_(average) =Σx[n] ² /T  (Equation 5)

Subsequently, equation 5 may be reduced as shown in Equation 6, where:Power_(average) =N*STM ² /T=N*STM ²/(N/f _(c))=f _(c) *STM ²  (Equation6)

FIG. 5 schematically shows a MEMS system according to an illustrativeembodiment. The MEMS gyroscope sensor 402 may have an operatingfrequency (f₀) of 72 KHz, such as the MEMS gyroscope series no. ADxrs290designed by Analog Devices, Inc. The bandwidth of the band pass filtermay be specified as center frequency f₀/16, thus the pass band isbetween 69.75 KHz and 74.25 KHz where f₀−2.25 KHz<B_(freq)<f₀+2.25 KHz.The chipping frequency of the pseudo-random number generator may be 2.25KHz (i.e., f₀/32). Here, the self-test signal detection portion mayemploy a cascade filter 502, such as a cascaded integrator-comb filter(CIC). The cascade filter 502 may have an integrator 504 and adifferentiator 506. The integrator 504 may operate at sampling rate 2*f₀and the differentiator 506 may operate at f₀/32.

Generally, cascade filters have a series of chained filters that may becoupled with rate changers (i.e., decimators and interpolators). Cascadefilters may employ decimation techniques to allow for a shorter lengthnumber sequence (n). As such, simpler circuit implementation may bedesigned. For example, for a signal test signal having 1 deg/s and arate of 30 deg/s, the power may be 1/30^(th) the magnitude of theCoriolis effect (i.e., 20 log₁₀(30)=29.5). Accordingly, where a sequenceof n>1023 may originally be necessary, the addition of a third-orderlow-pass filter with decimation may allow a length n=127 to be used. Thethird-order low pass filter may be implemented, for example, as a CIClow-pass filter of 127 and 255 taps.

Compared to self-test circuits known in the art, the implementation of aself-test circuit that measures the self-test stimulus with amultiplier, a delay circuit, and a third-order low pass filter mayprovide a smaller die footprint. Additionally, the components may haveless stringent requirements (e.g., filter order, cut-off frequency andQ-factor) as compared to such known self-test circuits.

FIG. 6 is a plot illustrating the effect on spectrum power of varyinglength of the number sequence used to spread the self-test signal. Here,five power spectrums are shown, including 4-bit LFSR (i.e., N=14sequence) 602, 5-bit LFSR (i.e., N=31 sequence) 604, 6-bit LFSR (i.e.,N=63 sequence) 606, 7-bit (i.e., N=127 sequence) 608, and 8-bit (i.e.,N=255 sequence) 610. As shown, with each additional bit to the sequence,the power spectrum spreads twice as much, thus the spread has a lowerfrequency component of approximately 3 dB (decibels). Thus, the lengthof the number sequence may be configured to produce a specific frequencycomponent magnitude.

FIG. 7 is a plot illustrating the effect of the magnitude of theself-test signal on the power spectrum of the pseudo-random numbergenerator. Here, the x-axis shows a frequency range from 69.5 KHz to74.5 KHz and the y-axis shows a magnitude of the power spectrum. Thefigure shows power spectrum of the output of the band-pass filter (Hannwindowed) for various values of magnitude levels of the self-testsignal, including “0.1” 702, “0.2” 704, “0.3” 706, “0.4” 708, “0.5” 710,“0.6” 712, and “0.75” 714. A value of “0.1” corresponds, for example, tothe self-test signal being 10% of magnitude of the stimulus 104.

From the plot, if the amplitude of a sinusoidal signal is changed by afactor of two, power changes by a factor of four. Power is a logfunction, where 20*log 10|FFT| peak will produce a change of 6 dB.Similarly when the STM increases by a factor of two, the FFT peaks ofthe self-test signal 114 (e.g., PN sequence*STM) increases so that theoverall power quadruples. When the amplitude is changed by a factor ofthree, it is observed that the resulting change in power is 9.5 dB.Here, the pseudo-random number has a sequence where N=31 and chippingfrequency f_(c)=2.25 KHz. It is noted that a decrease in the self-testmagnitude by half results in a FFT-magnitude reduction by a factor offour. As indicated, total power in the signal is proportional toΣ|FFT|².

Table 2 illustrates an effect of different self-test magnitude onself-test response. As observe, the resulting response may be higher orlower than the input magnitude of the self-test signal. The coriolisrate may be modeled as a, rate=bias+A*sin(2πft). Here, a bias of 30deg/s is assumed and a stimulus of interest (A) input of zero.

TABLE 2 Effect of self-test signal magnitude on Self-test responseCoriolis rate = Bias + A * sin(2 * STM pi * f * t), f = 100 Hz STMrecovered S. No (deg/s) Bias (deg/s) A (deg/s) (deg/s) 1 0.5 30 0 0.6 21 30 0 0.95 3 2 30 0 1.66

FIG. 8 is a plot illustrating the effect of the chipping frequency,f_(c), on the pseudo-random number generated sequence. Here, the x-axisshows a frequency range from 0 to 500 Hz and the y-axis shows amagnitude of the power spectrum between 0 and 50. The pseudo-randomnumber has a sequence where N=31 and chipping frequency f_(c)=125 Hz(802), 250 Hz (804), and 500 Hz (806).

FIG. 9 is a plot illustrating the power of the spectrum of thepseudo-random number generated sequence of FIG. 8. It is observed thatthe power remains the same over N for the same chipping frequency(f_(c)).

FIGS. 10A-B illustrate plots of the synchronization of thevarying-frequency self-test signal at the receiver. FIG. 10A shows atime domain output of the output signal 414 and the pseudo-random numbersequence 410. As shown, there exists a time offset 1002.

FIG. 10B shows a time-domain output of the output signal 414 and thein-phase pseudo-random number sequence 411. Here, the pseudo-randomnumber sequence is delayed by 0.4558 ms. Any of various types of phasesynchronization techniques may be employed, including, for example,phase-lock loops or correlation filters. The phase synchronization maybe fixed, varying, or adaptive.

FIG. 11 is a plot of an output of a low-pass filter with different tapconfiguration. Here, the final demodulated self-test response at theoutput of LPF is a constant close to 1 deg/s. The line 1102 shows theoutput for a third-order CIC filter with 127 taps (rM). The line 1104shows the output for a third-order CIC filter with 255 taps (rM).

FIG. 12 schematically illustrates an example of a MEMS sensor withself-test capabilities as known in the art. As indicated, a self-teststimulus may be a periodic single tone signal (e.g., a sinusoid). Sincea MEMS gyroscope also senses rotation simultaneously, the sensor outputhas two components, one due to rotation and the other due to self-teststimulus. The self-test component is separated from rotation componentand is monitored continuously. Separating the single tone self-test fromthe baseband rotation rate signal may be implemented using a band passfiltering 1202. To separate the rotation rate (stimulus) from theself-test (self-test stimulus), multi-stage low pass filtering may needto be performed. This low pass filter should have a notch at a specificpoint where the single tone self-test resides. Thus, at least one BPF isin the self-test path, and at least one LPF is in the self-test path.This design has certain practical difficulties in the design, such asstringent filtering requirements to separate the self-test component(self-test stimulus) from the rotation rate component (stimulus), aswell as vague criteria for the choice of an appropriate single tonefrequency. Additionally, the usable sensor bandwidth for measuring therotation rate (stimulus) is trade-off with the self-test stimulus, thustaking away from the effective measurable bandwidth of the sensor.

FIG. 13 is a flow chart of a method according to the illustrativeembodiment. A self-test signal having varying-frequency bands isinjected into a MEMS sensor (step 1302. The varying frequency self-testsignal is filtered from a sensor signal from the MEMS sensor (step1310). The injected self-test signal is compared to the filtered varyingfrequency self-test signal to determine the health of the MEMS sensor(step 1320).

The self-test operation may be initialized as the MEMS sensor isinitialized. Alternatively, the self-test operation may operateindependently of the MEMS sensor operation. For example, the MEMS sensormay be configured with a test mode in which the self-test signal may besolely injected into the MEMS sensor.

It should be apparent to one skilled in the art that the variousembodiments may be applied to other types of MEMS sensors, including,for example, accelerometers, microphones, and pressure sensors.

It should be noted that arrows may be used in drawings to representcommunication, transfer, or other activity involving two or moreentities. Double-ended arrows generally indicate that activity may occurin both directions (e.g., a command/request in one direction with acorresponding reply back in the other direction, or peer-to-peercommunications initiated by either entity), although in some situations,activity may not necessarily occur in both directions. Single-endedarrows generally indicate activity exclusively or predominantly in onedirection, although it should be noted that, in certain situations, suchdirectional activity actually may involve activities in both directions(e.g., a message from a sender to a receiver and an acknowledgement backfrom the receiver to the sender, or establishment of a connection priorto a transfer and termination of the connection following the transfer).Thus, the type of arrow used in a particular drawing to represent aparticular activity is exemplary and should not be seen as limiting.

It should also be noted that logic flows may be described herein todemonstrate various aspects of the invention, and should not beconstrued to limit the present invention to any particular logic flow orlogic implementation. The described logic may be partitioned intodifferent logic blocks (e.g., programs, modules, functions, orsubroutines) without changing the overall results or otherwise departingfrom the true scope of the invention. Often times, logic elements may beadded, modified, omitted, performed in a different order, or implementedusing different logic constructs (e.g., logic gates, looping primitives,conditional logic, and other logic constructs) without changing theoverall results or otherwise departing from the true scope of theinvention.

Aspects of the present invention may be embodied in many differentforms, including, but in no way limited to, computer program logic foruse with a processor (e.g., a microprocessor, microcontroller, digitalsignal processor, or general purpose computer), programmable logic foruse with a programmable logic device (e.g., a Field Programmable GateArray (FPGA) or other PLD), discrete components, integrated circuitry(e.g., an Application Specific Integrated Circuit (ASIC)), or any othermeans including any combination thereof. Computer program logicimplementing some or all of the described functionality is typicallyimplemented as a set of computer program instructions that is convertedinto a computer executable form, stored as such in a computer readablemedium, and executed by a microprocessor under the control of anoperating system. Hardware-based logic implementing some or all of thedescribed functionality may be implemented using one or moreappropriately configured FPGAs. For example, the self-test circuits 112and 212 may be implemented solely in hardware or may be implemented witha combination of hardware and software. The signal processing ASIC 304is generally implemented solely in hardware.

Computer program logic implementing all or part of the functionalitypreviously described herein may be embodied in various forms, including,but in no way limited to, a source code form, a computer executableform, and various intermediate forms (e.g., forms generated by anassembler, compiler, linker, or locator). Source code may include aseries of computer program instructions implemented in any of variousprogramming languages (e.g., an object code, an assembly language, or ahigh-level language such as Fortran, C, C++, JAVA, or HTML) for use withvarious operating systems or operating environments. The source code maydefine and use various data structures and communication messages. Thesource code may be in a computer executable form (e.g., via aninterpreter), or the source code may be converted (e.g., via atranslator, assembler, or compiler) into a computer executable form.

Computer program logic implementing all or part of the functionalitypreviously described herein may be executed at different times on asingle processor (e.g., concurrently) or may be executed at the same ordifferent times on multiple processors and may run under a singleoperating system process/thread or under different operating systemprocesses/threads. Thus, the term “computer process” refers generally tothe execution of a set of computer program instructions regardless ofwhether different computer processes are executed on the same ordifferent processors and regardless of whether different computerprocesses run under the same operating system process/thread ordifferent operating system processes/threads.

The computer program may be fixed in any form (e.g., source code form,computer executable form, or an intermediate form) either permanently ortransitorily in a tangible storage medium, such as a semiconductormemory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-ProgrammableRAM), a magnetic memory device (e.g., a diskette or fixed disk), anoptical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card),or other memory device. The computer program may be fixed in any form ina signal that is transmittable to a computer using any of variouscommunication technologies, including, but in no way limited to, analogtechnologies, digital technologies, optical technologies, wirelesstechnologies (e.g., Bluetooth), networking technologies, andinternetworking technologies. The computer program may be distributed inany form as a removable storage medium with accompanying printed orelectronic documentation (e.g., shrink wrapped software), preloaded witha computer system (e.g., on system ROM or fixed disk), or distributedfrom a server or electronic bulletin board over the communication system(e.g., the Internet or World Wide Web).

Hardware logic (including programmable logic for use with a programmablelogic device) implementing all or part of the functionality previouslydescribed herein may be designed using traditional manual methods, ormay be designed, captured, simulated, or documented electronically usingvarious tools, such as Computer Aided Design (CAD), a hardwaredescription language (e.g., VHDL or AHDL), or a PLD programming language(e.g., PALASM, ABEL, or CUPL).

Programmable logic may be fixed either permanently or transitorily in atangible storage medium, such as a semiconductor memory device (e.g., aRAM, ROM, PROM, EEPROM, or Flash-Programmable RAM), a magnetic memorydevice (e.g., a diskette or fixed disk), an optical memory device (e.g.,a CD-ROM), or other memory device. The programmable logic may be fixedin a signal that is transmittable to a computer using any of variouscommunication technologies, including, but in no way limited to, analogtechnologies, digital technologies, optical technologies, wirelesstechnologies (e.g., Bluetooth), networking technologies, andinternetworking technologies. The programmable logic may be distributedas a removable storage medium with accompanying printed or electronicdocumentation (e.g., shrink wrapped software), preloaded with a computersystem (e.g., on system ROM or fixed disk), or distributed from a serveror electronic bulletin board over the communication system (e.g., theInternet or World Wide Web). Of course, some embodiments of theinvention may be implemented as a combination of both software (e.g., acomputer program product) and hardware. Still other embodiments of theinvention are implemented as entirely hardware, or entirely software.

The present invention may be embodied in other specific forms withoutdeparting from the true scope of the invention, and numerous variationsand modifications will be apparent to those skilled in the art based onthe teachings herein. Any references to the “invention” are intended torefer to exemplary embodiments of the invention and should not beconstrued to refer to all embodiments of the invention unless thecontext otherwise requires. The described embodiments are to beconsidered in all respects only as illustrative and not restrictive.

What is claimed is:
 1. A MEMS sensor having a runtime self-test circuit,the MEMS sensor comprising: a micro-electromechanical device thatoutputs, during runtime operation, a device output signal; a detectioncircuit configured to detect, during the runtime operation of themicro-electromechanical device, a stimulus produced by themicro-electromechanical system in a sensor bandwidth of the deviceoutput signal, the stimulus having a magnitude above a predetermineddetection threshold of the detection circuit, the detection circuitfurther configured to produce a sensor output signal based on thedetected stimulus; and a self-test circuit configured to: inject, duringthe runtime operation of the micro-electromechanical device, a self-testsignal into a signal path of the micro-electromechanical deviceincluding a MEMS structure involved with generation of the stimulus inthe sensor bandwidth of the device output signal, wherein the self-testsignal is a spread-spectrum signal having a test-signal bandwidth thatoverlaps at least a portion of the sensor bandwidth; detect a testsignal component in the test-signal bandwidth of the device outputsignal based on a reference test signal corresponding to the injectedself-test signal, the test signal component have a magnitude below thedetection threshold of the detection circuit; and produce a test outputsignal indicating a status for the micro-electromechanical system basedon the test signal component and the reference test signal.
 2. A MEMSsensor according to claim 1, wherein the self-test circuit is configuredto continuously inject the self-test signal into themicro-electromechanical device during the runtime operation of themicro-electromechanical device.
 3. A MEMS sensor according to claim 1,wherein the self-test circuit comprises: a pseudo-random number sequencesource configured to provide a pseudo-random number sequence; and amodulation circuit configured to modulate the pseudo-random numbersequence and a self-test magnitude reference to produce a modulatedsignal and to inject the modulated signal into themicro-electromechanical device.
 4. A MEMS sensor according to claim 3,wherein at least one of: the self-test magnitude reference is a constantdirect-current (DC) source; the pseudo-random number sequence sourcecomprises a pseudo-random number generator; the pseudo-random numbersequence source comprises a memory in which is stored the pseudo-randomnumber sequence; the bandwidth of the self-test signal is equal to thesensor bandwidth; the bandwidth of the self-test signal is less than thesensor bandwidth; the bandwidth of the self-test signal is greater thanthe sensor bandwidth; the bandwidth of the self-test signal extendsbelow the sensor bandwidth; or the bandwidth of the self-test signalextends above the sensor bandwidth.
 5. A MEMS sensor according to claim3, wherein the self-test circuit further comprises: a demodulationcircuit configured to detect the test signal component by demodulatingthe device output signal with the reference test signal in-phase withtest signal component.
 6. A MEMS sensor according to claim 5, wherein atleast one of: the self-test circuit further comprises a low-pass filterto extract the test signal component from the device output signal; theself-test circuit further comprises a delay circuit to produce thereference test signal in-phase with the test signal component; theself-test circuit further comprises a memory to provide the referencetest signal; or the demodulation circuit comprises a multiplier tocombine the test signal component with the in-phase reference testsignal.
 7. A MEMS sensor according to claim 1, wherein the self-testcircuit is configured to detect the test signal component in thetest-signal bandwidth of the device output by: (i) correlating thesensor signal with the pseudo-random number sequence in phase with thesensor signal to produce a correlation signal; and (ii) comparing thecorrelation signal to at least one of a pre-defined correlationthreshold or a pre-defined signal energy threshold.
 8. A MEMS sensoraccording to claim 7, wherein the pre-defined energy threshold isestablished based on the relationship:fc*STM ² wherein fc is a frequency value of the injected self-test testsignal and STM is an average power of a period spread of the injectedself-test signal.
 9. A MEMS sensor according to claim 1, wherein theself-test circuit is configured to inject the self-test signal accordingto at least one of a frequency-hopping spread spectrum modulation, adirect-sequence spread spectrum (DSSS) modulation, a time-hopping spreadspectrum modulation, or a chirp spread spectrum modulation.
 10. A MEMSsensor according to claim 1, wherein the micro-electromechanical deviceincludes at least one of: an inertial sensor; a sound sensor; or apressure sensor.
 11. A self-test circuit for a micro-electromechanicalsystem that outputs, during runtime operation, a device output signalincluding a stimulus in a sensor bandwidth of the device output signal,the stimulus having a magnitude above a predetermined detectionthreshold, the self-test circuit comprising: a self-test signalgenerator configured to inject, during runtime operation of themicro-electromechanical device, a self-test signal into a signal path ofthe micro-electromechanical device including a MEMS structure involvedwith generation of the stimulus in a sensor bandwidth of the deviceoutput signal, wherein the self-test signal is a spread-spectrum signalhaving a test-signal bandwidth that overlaps at least a portion of thesensor bandwidth; a self-test signal detector configured to detect atest signal component in the test-signal bandwidth of the device outputsignal based on a reference test signal corresponding to the injectedself-test signal, the test signal component have a magnitude below thedetection threshold of the detection circuit; and a controllerconfigured to produce a test output signal based on the detected testsignal component.
 12. A self-test circuit according to claim 11, whereinthe self-test signal generator is configured to continuously inject theself-test signal into the micro-electromechanical device during theruntime operation of the micro-electromechanical device.
 13. A self-testcircuit according to claim 11, wherein the self-test signal generatorcomprises: a pseudo-random number sequence source configured to providea pseudo-random number sequence; and a modulation circuit configured tomodulate the pseudo-random number sequence and a self-test magnitudereference to produce a modulated signal and to inject the modulatedsignal into the micro-electromechanical device.
 14. A self-test circuitaccording to claim 13, wherein at least one of: the self-test magnitudereference is a constant direct-current (DC) source; the pseudo-randomnumber sequence source comprises a pseudo-random number generator; thepseudo-random number sequence source comprises a memory in which isstored the pseudo-random number sequence; the bandwidth of the self-testsignal is equal to the sensor bandwidth; the bandwidth of the self-testsignal is less than the sensor bandwidth; the bandwidth of the self-testsignal is greater than the sensor bandwidth; the bandwidth of theself-test signal extends below the sensor bandwidth; or the bandwidth ofthe self-test signal extends above the sensor bandwidth.
 15. A self-testcircuit according to claim 11, wherein the self-test signal detectorcomprises: a demodulation circuit configured to detect the test signalcomponent by demodulating the device output signal with the referencetest signal in-phase with test signal component.
 16. A self-test circuitaccording to claim 15, wherein at least one of: the self-test signaldetector comprises a low-pass filter to extract the test signalcomponent from the device output signal; the self-test signal detectorcomprises a delay circuit to produce the reference test signal in-phasewith the test signal component; the self-test signal detector comprisesa memory to provide the reference test signal; or the demodulationcircuit comprises a multiplier to combine the test signal component withthe in-phase reference test signal.
 17. A self-test circuit according toclaim 11, wherein the self-test signal detector is configured to detectthe test signal component in the test-signal bandwidth of the deviceoutput by: (i) correlating the sensor signal with the pseudo-randomnumber sequence in phase with the sensor signal to produce a correlationsignal; and (ii) comparing the correlation signal to at least one of apre-defined correlation threshold or a pre-defined signal energythreshold.
 18. A self-test circuit according to claim 17, wherein thepre-defined energy threshold is established based on the relationship:fc*STM ² wherein fc is a frequency value of the injected self-test testsignal and STM is an average power of a period spread of the injectedself-test signal.
 19. A self-test circuit according to claim 11, whereinthe self-test circuit is configured to inject the self-test signalaccording to at least one of a frequency-hopping spread spectrummodulation, a direct-sequence spread spectrum (DSSS) modulation, atime-hopping spread spectrum modulation, or a chirp spread spectrummodulation.
 20. A method of evaluating the status of a MEMS sensorduring runtime, the MEMS sensor having a micro-electromechanical devicethat outputs, during a runtime operation, a device output signal, theMEMS sensor further having a detection circuit configured to detect,during the runtime operation of the micro-electromechanical device, astimulus produced by the micro-electromechanical system in a sensorbandwidth of the device output signal, the stimulus having a magnitudeabove a predetermined detection threshold of the detection circuit, thedetection circuit further configured to produce a sensor output signalbased on the detected stimulus, the method comprising: injecting, duringruntime operation of the micro-electromechanical device, a self-testsignal into a signal path of the micro-electromechanical deviceincluding a MEMS structure involved with generation of a stimulus in asensor bandwidth of a device output signal, wherein the self-test signalis a spread-spectrum signal having a test-signal bandwidth that overlapsat least a portion of the sensor bandwidth; detecting a test signalcomponent in the test-signal bandwidth of the device output signal basedon a reference test signal corresponding to the injected self-testsignal, the test signal component have a magnitude below the detectionthreshold of the detection circuit; and producing a test output signalbased on the detected test signal component.